This invention relates to nuclear steam generators and more particularly to apparatus for remotely repairing the tubes and tube sheet in a nuclear steam generator.
There are many situations in which a hazardous environment limits human access to various locations. One such situation occurs in the inspection and repair of nuclear steam generators which have been operating. A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tube sheet forming a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The primary fluid having been heated by circulation through the nuclear reactor core enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, primary fluid flows upwardly through first openings in the U-tubes near the tube sheet which supports the tubes, through the U-tube curvature, downwardly through second openings in the U-tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-tubes in heat transfer relationship therewith thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-tube walls and the tube sheet, it is important that the U-tubes and the tube sheet be maintained defect-free so that no breaks will occur in the U-tubes or in the welds between the U-tubes and the tube sheet thus preventing contamination of the secondary fluid by the primary fluid.
Occasionally it is necessary to either inspect or repair the U-tubes or tube sheet welds by way of access through the primary fluid inlet and outlet plena. For this purpose manways are provided in the vertical shell so that working personnel may enter the inlet and outlet plena to perform operations on the U-tubes and tube sheet. However, since the primary fluid which is generally water contains radioactive particles, the inlet and outlet plena become radioactive which thereby limits the time that working personnel may be present therein. Accordingly, it would be advantageous to be able to perform operation on the U-tubes and tube sheet without requiring the presence of working personnel. There are several mechanisms known in the art that attempt to provide a solution to this problem, but none of them have been able to completely solve the problem.
In copending application Ser. No. 888,701, filed Mar. 21, 1978, and entitled "Apparatus For Remotely Repairing Tubes In A Steam Generator" by L. R. Golick and assigned to the assignee of the present application, there is described a positioning mechanism for remotely positioning repair tools with respect to the U-tubes and tube sheet of a nuclear steam generator. The apparatus comprises a column rotatably disposed in either the inlet or outlet plenum of a nuclear steam generator with the column having one end terminating a predetermined distance from the tube sheet while being fastened to the divider plate and with the bottom end of the column resting in a spherical bearing block. The apparatus also comprises a boom pivotally mounted near the top end of the column with a carriage disposed on the boom and capable of moving along the boom for positioning a tool that is mounted on the carriage in relationship to the tube sheet or the U-tubes of the nuclear steam generator to thereby perform operations on the nuclear steam generator.
Copending application Ser. No. 895,831, filed Apr. 13, 1978 and entitled "Improved Apparatus For Remotely Repairing Tubes In A Steam Generator" describes an improved positioning mechanism for remotely positioning repair tools in a steam generator. The improved positioning mechanism includes a cantilevered tool holder having a chuck on the end thereof along with a drive mechanism connected to the chuck for providing power to a tool mounted in the chuck.
While these copending applications do describe positioning mechanisms for positioning tools with respect to the tube sheet and U-tubes of a nuclear steam generator, what is needed is a drill that is capable of being attached to the tube sheet or U-tubes in the nuclear steam generator to thereby partially support the weight of the drill tool, resist the drilling loads imposed on the boom, and to guide the drill tool during the operations.